1. Field of the Invention
The invention relates to a light emitting system, an optical power control device, and a control signal module.
2. Description of the Related Art
FIG. 1 shows a conventional optical power control device adapted to receive a direct-current input voltage and generate a working current to drive a light-emitting diode (LED) 1. When the input voltage is constant, the working current has a constant magnitude.
However, the conventional optical power control device has the following drawbacks:
1. The working current resulting from the direct-current input voltage will increase temperature of the LED 1, and characteristics of the LED 1 will vary with temperature.
2. Referring to FIG. 2, a forward voltage of the LED 1 varies with ambient temperature, and LEDs 1 with different colors (e.g., blue, green and red) follow different forward voltage-temperature curves. When the LED 1 is driven with a constant current (e.g., 20 mA), rise of the ambient temperature may result in drop of the forward voltage, so that the output power of the LED 1 (=forward voltage×working current) drops with rise of the ambient temperature.
3. In application, several LEDs 1 with different colors are frequently used together to obtain light with a desired color temperature and a desired color rendering index. When each of the LEDs 1 with different colors is driven by a corresponding conventional optical power control device, the power ratio thereamong may drift due to different drop levels among the LEDs 1, such that the desired color temperature and the desired color rendering index may not be maintained.